The present invention relates to digital timing generation, and more particularly to digital timing generation using a state machine to identify a specific pulse pattern within an input signal from which the presence of a valid input signal may be determined.
In many applications electrical signals occur which have an identification pulse pattern which occurs at a periodic rate. One such signal is the composite sync signal of a television signal which has a definite pulse pattern to identify the vertical sync portion of the composite sync signal, i.e., the identification of a new field which requires a vertical retrace to the top of the display. Detection of such a defined pulse pattern can be used to determine the presence or absence of a valid input signal.
Prior techniques have used discrete components such as non-retriggerable one-shot multivibrators and counters to separate the vertical sync pulses from a television composite sync signal. The sync pulses are then counted to assure that the appropriate number of sequential pulses is present to identify that a valid television signal is present. The identified vertical sync pulse is then used to provide the necessary vertical timing within a television system.
Alternatively complex flip-flop circuitry realized in the form of application specific integrated circuits (ASIC) has been used to perform the same function of pulse width determination based upon counting clock pulses and then counting the number of pulses of the proper width to identify a valid vertical sync pulse which is then used to identify that a valid television signal is present.
Although the prior techniques work adequately, they do not provide flexibility to cope with a noisy input signal, and are somewhat complex. What is desired is a digital timing generator which has high noise immunity, greater flexibility and is relatively simple to implement.